This invention relates to electrical testing of data transmission cables and more particularly, to apparatus for automatically matching the impedance between the test instrumentation and a cable under test.
With the proliferation of local area networks (LAN's) for data communication, it is important during the installation and maintenance of the network, to be able to test the data transmission cables being used quickly and efficiently. Data transmission cables used in LAN's are either coaxial cables or twisted pair cables. To facilitate testing of both types of cable, a tester has been developed which permits both automated testing of a cable for such parameters as characteristic impedance, down-line impedance, overlimit values, and cable length; as well as allowing the user to perform a selected, individual test. A problem involved in the development of this instrument is that coaxial and twisted pair cables have a wide range of cable impedances; these values ranging, for example, from 50-ohms to 150-ohms.
Cables may be tested in a number of ways, one of these being the use of time domain reflectometry (TDR). In this technique, a signal, usually a pulse, is transmitted down the cable and the reflected pulse analyzed. If the signal source is not impedance matched to the cable under test, multiple reflections of the test signal will occur. This results in incorrect length and impedance measurements. One method of impedance matching is to manually switch separate baluns in and out of the circuit path between the signal source and the cable under test. This trial-and-error method will ultimately produce an impedance match; however, it is time consuming and does not lend itself to the other automatic features incorporated in the test instrument.